Carbon fillers are used as color pigments for plastics, textiles, paints and the like, centering around applications as reinforcing fillers for rubber products. Attempts have also been being made to apply them in IC trays, electronic parts materials and the like, as their mixing in plastics makes it possible to impart electrical conductivity. In recent years, their applications have also been under study in various fields, including studies on carbon nanotube thin films as replacements for transparent electrodes (ITO films) in FPDs (flat panel displays) such as liquid crystal panels and organic EL panels.
A carbon filler generally features small primary particles and high cohesive power, and therefore, is a material that can be hardly dispersed uniformly. There is, accordingly, an outstanding demand for a dispersant that enables more uniform dispersion. In particular, the uniform dispersion of a carbon filler in an aqueous solvent containing water and a hydrophilic organic solvent is difficult, because the carbon filler is hydrophobic at surfaces thereof and hence has low surface energy and poor wettability to the aqueous solvent.
A variety of problems arise unless the dispersion state of a carbon filler is uniform. In applications such as paints and printing inks, there is a problem that neither vividness nor gloss can be obtained as intended. Especially in the application to electrically conductive materials, an adequate conductive network can be hardly formed to possibly produce inconvenience in a fundamental characteristic such that an intended electrical resistance cannot be reached. For allowing a carbon filler to fully exhibit its functionality, it thus becomes necessary that the state of a slurry is appropriate for the exhibition of the functionality, specifically that the carbon filler is uniformly and stably dispersed in the slurry.
Choosing adequate solvents with a focus being centered around the dispersibility of a carbon filler, nonaqueous (organic-solvent-based) solvents (dispersion media), which are excellent in the uniform dispersibility of the carbon filler, show high adhesive force and can be readily dried, are overwhelmingly advantageous as solvents (dispersion media) for the slurry. As a matter of fact, nonaqueous solvents have hence been used widely (see Non-patent Documents 1 and 2, etc.).
However, nonaqueous organic solvents are not only volatile and high in environmental load but are also required to take genotoxicity into consideration, and therefore, still involve problems in safety and workability. It is hence desired to use them in reduced amounts. In recent years, there is an increasing concern about the protection of environment and the prevention of health hazards in many industrial fields (Non-patent Document 3), leading to increasing demands for measures toward VOC reductions, solventless coating and the like in connection with the use of nonaqueous organic solvents involving such problems as described above. It is, accordingly, required to switch to products that are friendly to the environment and people (Non-patent Publication 4).
Those which are attracting the greatest attention as products friendly to the environment and people are water-based products, which are expected to become part of solventless products. Various problems, however, arise if water is used as a solvent in place of an organic solvent in a slurry that contains a carbon filler. In a water-based slurry, for example, carbon filler particles tend to agglomerate in the slurry when they are in a charged state, and moreover, they are prone to settling due to a large difference in specific gravity between the solvent and the solute, thereby raising a problem in that their uniform dispersion is very difficult (Non-patent Document 1).
There is, accordingly, an urgent need to take measures for the assurance of good dispersion for a carbon filler. As general measures to cope with dispersion failures, addition of a dispersant, surface treatment or microencapsulation of a carbon filler, introduction of polar groups into a polymer, and the like can be contemplated. In fact, proposed as to the addition of a dispersant include the attempt to use a water-soluble, amphoteric dispersant for a slurry composition that contains a microparticulated black inorganic oxide useful in paints, inks, rubbers and plastics, electronic materials and the like (Patent Document 1) and the attempt to use a compound having one or more basic functional groups in an composition for cells, which contains a conductive aid (Patent Document 2). Concerning the application of surface treatment to a carbon filler, there are proposals about carbon black with a surface acidity increased by performing gas-phase oxidation treatment such as ozone treatment or plasma treatment or liquid-phase oxidation treatment with hydrogen peroxide solution, sodium perchlorate or the like such that hydrophilic functional groups such as carboxyl groups or hydroxyl groups are introduced onto the surfaces of the carbon black (Patent Documents 3 and 4).
Other proposals include the conversion of a conductive filler into a microencapsulated conductive filler by forming an insulating resin on the surfaces of the conductive filler, and further, an anisotropic conductive resin composition containing a polymer having polar groups and a conductive filler.
However, the dispersion media used in these proposals are primarily organic solvents, and water-based media are used only in a very small number of cases. The carbon black with the hydrophilic functional groups introduced by the surface treatment is provided with increased wettability to an aqueous solvent, and improvements are observed in its water dispersibility. However, the hydrophilic functional groups so introduced act as a disincentive for electrical conductivity so that limitations are imposed including its unsuited use in application fields where electrical conductivity is intended. It is, therefore, strongly desired to develop a method that uses a water-based slurry, which, even if a nonaqueous organic solvent is used, contains the nonaqueous organic solvent in a small proportion, is friendly to the environment, is low cost and is high in safety, and that can uniformly disperse a carbon filler.
Upon attempting the dispersion stabilization of a slurry with a carbon filler dispersed in an aqueous solvent, the use of the above-described, respective methods for organic solvents may be contemplated. Among them, the method involving “the addition of a dispersant” is advantageous when the simplification of the production process and coating system and the cost mat ter are taken into account. As a dispersant for use in a water-based slurry, it is possible to mention a polycarboxylate salt or phosphate amine salt used in the field of paints, a polyacrylamide as a high-molecular dispersant, or the like. When a reduction in environmental load is taken into consideration, however, preferred is a substance of natural origin for its friendliness to the environment. As such a natural substance, it has been proposed to use carboxymethylcellulose as a water-based dispersant upon production of each electrode for a nonaqueous electrolyte secondary cell (Patent Document 4). However, there is still a room for an improvement in its dispersing effect.